For several decades numerous processes have been known in the art to prepare various monomer emulsions through which numerous polymer compounds have been produced based on homopolymers, copolymers and terpolymers that involve vinyl monomers and/or conjugated dienes. The commonly used components in emulsion polymerization are monomers which are dispersed in water through emulsifiers, that can be ionic, non-ionic or amphoteric, dispersing agents, transfer agents, pH regulators, electrolytes, inhibitors and initiator. The initiator can be water soluble or organic phase soluble. In order to reduce the half life time of the initiator, reducing agents can be used.
The processes can be defined in several ways. The oldest ones were batch processes classified as cold or hot processes. The cold processes are carried out at low temperatures (about 5 degrees C.) and the hot processes are carried out at a reaction temperature above 50 degrees C. In some cases, the reaction must be stopped within low conversion ranges (from 50 to 75%) in order to obtain appropriate rubber characteristics. The batch processes are the processes in which all the ingredients are charged at the beginning of the reaction. There are semi-batch processes, in which some of the ingredients are charged at the beginning of the reaction and the remaining part of the ingredients is added at predetermined times.
The continuous processes consist of an input stream of polymerization ingredients and an output stream of the product. In the case of the continuous processes, a reactor cascade is usually required to achieve high conversion rates.
In the past few years, environmental regulations have been focusing their attention to the reduction of residual monomers and the so-called organic volatile compounds such as styrene, acrylonitrile, vinyl chloride, etc.
Independently of the reaction process, the final monomer conversion into polymer is never 100%, which implies that there is residual monomer occluded in the emulsion polymer particles.
Traditionally, the residual monomer removal is carried out stripping said monomer through vacuum and/or steam. Among the disadvantages related to the use of stripping, there is the fact that it is a highly energy consuming process (vacuum and steam). The equipment in which the stripping is carried out fouls itself considerably which causes productivity reduction, together with pollution problems caused by the material adhered to the equipment.
During the vacuum application, foam is generated and to prevent foam formation antifoam agents are added which negatively affect the functional characteristics of the latex (film formation, solvent resistance, adhesiveness, brightness, etc.). There is also the polymer destabilization and degradation problem due to the large periods of time required for the stripping. The water removed during the stripping contains residual monomer and latex, and this makes the treatment of residual water necessary in order to fulfill the ecological regulations that are becoming stricter every day.
For instance, in the U.S. Pat. No. 4,130,527 it is indicated that, in general, the organic volatile compounds are removed from the latex, using water steam, gases, or gas mixtures. In this technology it is established that the volatile compound removal speed is not controlled by the particle outlet diffusion speed, but by the speed at which the gas can contact the polymer particle to strip the volatile material to the aqueous phase and then to the gas phase. Some of the factors that affect the design of this type of equipment are: a) foam production during the process; b) colloidal instability resulting from temperature and mechanical stress, and c) the high viscosities caused by the product concentration. In the U.S. Pat. No. 4,529,753 it is stated that there are various types of devolatilizers commercially available, from the ones that operate in batches to the ones that operate in continuous processes and that have equipment cascades with counter-current or crossed gas flows. Another equipment that has been described is the one referring to stripping columns that form a fine latex film, under reduced pressure conditions, to increase the contact surface with the stripping gas (Monomer Removal from Latex, S.M. England publication, Dow Chemical Co. Midland, Mich. 48640 U.S.A.). It is obvious that these equipments are expensive and because of the process there is an important material loss caused by coagulation.
In the U.S. Pat. No. 4,301,264, an "Emulsion Polymerization Process" is described in which the oxidizing system is made up of hydroperoxides and the reducing agent system consists of amines. In this patent, the oxidizing agent is charged to the emulsion since the polymerization process.
A "steam and chemical stripping" method is also reported. In said method, low pressure conditions are used. The oxidizing system is a hydrogen peroxide, a diazo or azo compound and the reducing agent is a sulfur compound.